Motion sensors are used in security systems to detect movement in a monitored space. One type of sensor is a passive infrared (PIR) motion sensor, which detects changes in far infrared radiation (8–14 micron wavelength) due to temperature differences between an object (e.g. a human) and its background environment. Upon detection, motion sensors generally transmit an indication to a host system, which may in turn activate an intrusion “alarm”, change room lighting, open a door, or perform some other function.
One way to provide motion sensing capabilities is to provide an infrared camera. Motion in the monitored space can be tracked easily by observing the output of the camera. However, such cameras are expensive. Hence, the need for simple, relatively inexpensive PIR motion sensors, using, e.g., simple pyroelectric detectors. Because the detectors can be a significant part of the cost (5–10%) of a typical PIR motion sensor, most PIR motion sensors employ only one or two such detectors.
To monitor a large space with only one or two detectors, a typical PIR motion sensor is designed with multiple optical components (e.g. lenses or mirrors). Each component of such “compound optics” focuses the infrared radiation from objects within a respective sub-volume of the monitored space into an image appearing over the detector. The monitored sub-volumes can be interleaved with non-monitored sub-volumes, so that a radiation producing target (e.g., a human) passing from sub-volume to sub-volume causes a “target radiation/background radiation/target radiation” pattern at the detector. In the case of humans, this pattern causes changing IR radiation at the detector.
While effective, it happens that simple PIR sensors using a minimal number of detectors can generate false alarms from time to time, due, for example, to incident radiation of wavelength outside of the 8–14 micron band. Such false alarms may nonetheless precipitate unneeded responses by, e.g., security personnel. Accordingly, to reduce the likelihood of false alarms, optical filters have been added as detector windows to screen out white light and near IR light. Also, coatings (in the case of mirrors) and additives (for lenses) have been added to prevent the focusing of white and near infrared light onto detectors to reduce the possibility of PIR motion sensors producing false alarms due to, e.g., automobile headlights shining through windows.
To further reduce the chance of false alarms, detectors can include a pair of equally sized elements of opposing polarities. Non-focussed out-of-band radiation is equally incident on both elements, thus causing the signals from the equal and opposite elements to roughly cancel one another. Further, equal elements of opposite polarity also reduce false alarms from shock and temperature change. In addition, as disclosed in, e.g., U.S. Pat. No. 6,163,025, incorporated herein by references, two pair of elements can be interleaved and separately connected to generate motion signals that are shifted in time relative to one another. This facilitates differentiation between moving targets and stationary but otherwise problematic sources such as varying-intensity white lights.
The present invention recognizes, however, that the computational requirements for processing the time-shifted signals in the '025 patent are considerable. The present invention critically recognizes the need to reduce false alarms in simple PIR sensors while minimizing processing requirements. Moreover, it is recognized herein that it is desirable that a simple PIR motion sensor be capable of discriminating smaller moving targets, e.g., animals, from larger targets such as humans, so that an alarm will be activated only in the presence of unauthorized humans, not pets. The present invention addresses one or more of these critical observations.